Over 200 types of thistle plants exist in the world, with most of these in the northern hemisphere. Considered a weed by botanists, thistles are potentially invasive, especially the well-known Canada thistle, which can take over a large area. Most kinds of grazing animals ignore the thistles that develop in their pastures but one particularly colorful bird does not, a fact that can help you to identify the plant.
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Look for a tall plant. Most thistles are at least 2 feet high, and the Canada thistle can grow to be as tall as 5 to 6 feet.
Look for plants that grow in patches. Any part of the rootstock of a thistle has the ability to produce a new shoot, leading to large clusters in a given area. Common thistle habitats include waste places, old farm fields, pastures, fence rows and along the sides of country roads.
Recognize thistles by their leaves, which are oblong and possess irregularly shaped lobes. The defining characteristic of most thistle leaves is the sharp spines that protrude from the edges. Canada thistle leaves are dark green on top but a lighter shade of green underneath.
Feel the stems of a thistle and notice the hairs that grow on them. The immature plants have just a few of these hairs but the older ones have many.
Look for the colorful flowers that bloom on the top part of this weed from June through August. The thistle flower is typically some shade of purple but can also be white. The heads will range from 3/4 of an inch wide to almost 1 1/2 inches. While Canada thistle flowers lack bristles or any spines, other species of thistle have spines on the flower, making them difficult to pick.
Monitor pastures and fields for the activity of the colorful goldfinch. These birds will invariably lead you to thistles since they love to dine on the tiny seeds the plant produces. The goldfinch male is a brilliant yellow with black mixed in and you cannot miss one when it is flying low and landing on thistles.